Retroreflective sheeting has the ability to redirect light incident upon a major surface of the sheeting toward its originating source. This unique ability has led to the wide-spread use of retroreflective sheeting in a wide variety of conspicuity enhancement applications relating to traffic and personal safety marking. Typical examples of uses of retroreflective sheeting include the placement of such sheetings on road signs, traffic cones and barricades to enhance their conspicuity, particularly under poor lighting conditions, such as night-time driving conditions or in conditions of inclement weather. These uses typically allow the sheeting to be adhered to relatively flat and rigid surfaces, thereby allowing the sheeting to be relatively inflexible. Additionally, sign applications are characterized by relatively predictable, standardized viewing geometries.
There are essentially two types of retroreflective sheeting, beaded sheeting and cube corner sheeting. Beaded sheeting employs a multitude of independent glass or ceramic microspheres to retroreflect incident light. From the optics perspective, beaded sheeting typically exhibits favorable rotational symmetry and entrance angularity performance because of the symmetrical nature of the beads. Additionally, beaded sheeting typically exhibits relatively good flexibility because the beads are independent from one another. However, beaded sheeting tends to exhibit relatively low brightness when compared to cube corner sheeting.
Cube corner retroreflective sheeting typically employs an array of rigid, interconnected cube corner elements to retroreflect light incident on a major surface of the sheeting. The basic cube corner element is a generally tetrahedral structure having three mutually substantially perpendicular lateral faces which intersect at a single reference point, or apex, and a base triangle opposite the apex. The symmetry axis, or optical axis, of the cube corner element is the axis which extends through the cube apex and trisects the internal space of the cube corner element. In conventional cube corner elements which have an equilateral base triangle, the optical axis of the cube corner element is perpendicular to the plane which contains the base triangle. In operation, light incident on the base of the cube corner element is reflected from each of the three lateral faces of the element and is redirected toward the light source. Retroreflective sheeting generally incorporates a structured surface including at least one array of cube corner reflective elements to enhance the visibility of an object. When compared with beaded sheeting, cube corner retroreflective sheeting exhibits relatively greater brightness in response to light incident at relatively low entrance angles, for example, near normal light. However, cube corner retroreflective sheeting also exhibits relatively poor rotational symmetry performance at high entrance angles. In addition, cube corner retroreflective sheeting is typically stiffer than beaded sheeting because the cube corner elements are usually all interconnected.
The optics of cube corner retroreflective sheetings can be designed to exhibit optimal performance at a specific orientation. This can be accomplished by forming the cube corner elements of the retroreflective sheeting such that their optical axes are canted relative to an axis perpendicular to the base plane of the sheeting. U.S. Pat. No. 4,588,258, issued to Hoopman on May 13, 1986 ('258 Patent), the teachings of which are incorporated by reference herein, discloses retroreflective sheeting which employs optics having canted cube corner elements which form opposing matched pairs. The sheeting disclosed in the '258 Patent exhibits a primary plane of improved retroreflective performance at high entrance angles, identified as the x-plane in the '258 Patent, and a secondary plane of improved retroreflective performance at high entrance angles, identified as the y-plane in the '258 Patent. In use, it is recommended that sheeting manufactured with the '258 Patent be oriented such that its principal plane of improved retroreflective performance, for example the x-plane, is coincident with an expected entrance plane. Thus, sheeting in accordance with the '258 Patent has a single preferred orientation. The '258 Patent discloses cube-corner retroreflective articles in which the optical axis of the elements in an array of prism element pairs are tilted toward one edge of the elements, when considered from the front surface of the article on which light to be retroreflected images. This tilt direction is herein defined as "positive" type tilt.
In another patent, U.S. Pat. No. 2,380,447, issued to Jungersen on Jul. 31, 1945 ('258 Patent), the teachings of which are incorporated by reference herein, discloses in FIG. 15 of the '447 Patent the optical axis of prism pairs tilting away from the common edge.
In another patent, U.S. Pat. No. 5,171,624, issued on Dec. 15, 1992 to Walter, the teachings of which are incorporated herein in its entirety by reference) discloses microprism reflective sheeting in which prism pairs are tilted with respect to one another at an angle of in the range of between about three and ten degrees, prism size of 0.15-0.64 mm (0.006-0.025 inches) (space between apices) and wherein at least one prism side surface is arcuate.
However, a need still exists for a retroreflective structure that provides a more uniform distribution of light.